[ale] Raid perf?

[Greg Freemyer]

Jeff,

Raid 10 is slightly less redundant than Raid 6

If I have a Raid 10 with 6 drives and one fails I end up with 5 good
drives, but 1 of them is no longer redundant, so if it happened to
fail you lose the whole thing.

(Yes I'm talking about a dual disk failure where both disks happen to
be partners of each other.)

Raid 6 can function in dual drive failure, regardless of which 2 drives fail.

FYI: Raid 10 with large numbers of disks can handle even more than 2
failures if you don't lose any mirror pairs.  Raid 6 will always fail
if you have a 3 disk failure.

OTOH,
Raid 10 has a 50% waste of drives.

Raid 6 is only wasting 2 drives, so if you have 5 or more drives in
your array it has less waste.

The negative is the write speed.  To update a Raid 10, you just have
to write your update to two places.  i.e 2 i/o's per write.

With Raid 6, you apparently have to read the 3 drives worth of info to
figure out the original parity state, then write all 3 of those drives
with updated info.  Thus 6 i/o's for each write.


Greg

On Fri, Oct 24, 2008 at 5:58 PM, Jeff Lightner <jlightner at water.com> wrote:
> Other than performance is the fact that RAID10 can actually suffer TWO
> disk failures before the raid set goes away.   Haven't worked with RAID6
> so don't know its redundancy.
>
> -----Original Message-----
> From: ale-bounces at ale.org [mailto:ale-bounces at ale.org] On Behalf Of Greg
> Freemyer
> Sent: Friday, October 24, 2008 2:52 PM
> To: Atlanta Linux Enthusiasts
> Subject: Re: [ale] Raid perf?
>
> All,
>
> If anyone cares I found this Dell doc:
>
> http://www.dell.com/downloads/global/products/pvaul/en/PERC6_PerfWP_WMD1
> 120.pdf
>
> Per it, Raid 6 is in theory 3 times less efficient than Raid 10 on
> writes.
>
> So to maintain the same write performance as my 4-disk Raid 10 I would
> need a 12-disk Raid 6.
>
> That is worse than I realized, but feasible for my needs.  (I think
> 250GB drives are down to $60 or so.  Thus 12 is only $700.  Obviously
> I need the controller card too.  Haven't priced that yet.  I do have
> big chassis that can hold the drives.)
>
> Time for me to do a better job of characterizing my app to see what
> percentage the i/o's are reads vs. writes.
>
> Greg
>
> On Fri, Oct 24, 2008 at 9:53 AM, Greg Freemyer <greg.freemyer at gmail.com>
> wrote:
>> All,
>>
>> I have a Raid 10 (4x500GB) under Fedora using software raid that I
>> want to replace with a Raid 6 (6x1TB).
>>
>> Does anyone know a website that will help me understand what the
>> impact will be from a write perspective?  Assuming random DB style
>> writes, not sequential.
>>
>> I'm curious about both theoretical and real world  (3ware or linux
>> software raid) performance numbers.
>>
>> Thanks
>> Greg
>> --
>> Greg Freemyer
>> Litigation Triage Solutions Specialist
>> http://www.linkedin.com/in/gregfreemyer
>> First 99 Days Litigation White Paper -
>>
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> f
>>
>> The Norcross Group
>> The Intersection of Evidence & Technology
>> http://www.norcrossgroup.com
>>
>
>
>
> --
> Greg Freemyer
> Litigation Triage Solutions Specialist
> http://www.linkedin.com/in/gregfreemyer
> First 99 Days Litigation White Paper -
> http://www.norcrossgroup.com/forms/whitepapers/99%20Days%20whitepaper.pd
> f
>
> The Norcross Group
> The Intersection of Evidence & Technology
> http://www.norcrossgroup.com
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-- 
Greg Freemyer
Litigation Triage Solutions Specialist
http://www.linkedin.com/in/gregfreemyer
First 99 Days Litigation White Paper -
http://www.norcrossgroup.com/forms/whitepapers/99%20Days%20whitepaper.pdf

The Norcross Group
The Intersection of Evidence & Technology
http://www.norcrossgroup.com